The invention relates to a spent liquor recovery boiler comprising a water/steam circulation system having superheaters and reheaters, and a furnace for burning spent liquor to produce hot flue gases, and including an upper portion through which the flue gases flow. The water/steam circulation system is connected to a steam turbine comprising a high-pressure stage and a medium-pressure stage. Live steam superheating and steam reheating are performed utilizing an arrangement with which corrosion can be reduced.
In chemical pulp mills, the cooking chemicals of a pulping process are recovered from spent liquor, e.g., black liquor in kraft pulping, by firing the liquor in a recovery boiler either alone or together with other “waste” streams. The firing process is exothermic and the released energy is recovered as pressurized superheated steam. The steam energy is recovered in a steam turbine in the form of electric power and steam of different pressures for process needs. Kraft pulping which produces black liquor is the dominant pulping method and is used herein to describe the invention. The invention may be applied in various types of chemical pulp mills and other processes. While the term “black liquor” commonly used in the context of kraft pulping, it is also used herein to refer to all types of spent liquors.
Traditionally, energy is produced in a pulp mill by combusting black liquor in a recovery boiler, and wood wastes and bark in an auxiliary boiler. The bark of the wood raw material and the organic substance of generated black liquor together normally fulfill the entire energy demand of the pulp mill. If more energy is needed in the pulp mill, additional fuel may be purchased. The additional fuel is combusted with the wood bark in an auxiliary boiler. For example, a lime kiln may be fired with oil or with natural gas. At the present time, the production of power often takes place as follows: (i) the recovery boiler and the auxiliary boiler, in which the bark generated in the mill is combusted, produce superheated high-pressure steam; (ii) The produced steam is fed to one or more back pressure steam turbines and the steam from the turbine discharge provides heat for the mill, and (iii) The turbine and the generator connected thereto produce the electricity needed by the mill. Electricity is usually produced by a back pressure turbine having one or more bleedings. The back pressure used is 3-6 bar (abs.) and the bleed pressure 8-16 bar (abs.). The production of electricity may also be effected by means of a condensing turbine or by a condensing stage in the steam turbine, following the back pressure stage.
Wood contains small amounts of potassium (K) and chlorine (Cl). These elements remain in the black liquor during cooking. They may enter black liquor also via make-up chemicals, or via internal connections inside the mill. In the recovery boiler, these elements are enriched into the fly ash and increase the corrosiveness of the flue gas especially in the superheater.
The corrosiveness of Cl and K increase with temperature. The corrosiveness of Cl and K impose an upper temperature limit on the steam generated in the recovery boiler. This limit for the superheated steam temperature is typically 400° C. to 490° C., depending on the content of chlorine and potassium. With special materials or with liquors having a very low Cl and K content, either naturally or via fly ash dumping or via Cl and K removal process, steam temperatures up to 520° C. have been used. Because the corrosiveness of Cl and K generally require the temperature of the superheated steam to be held relatively low, the steam pressure is also low. These temperature limitations result in low power yield from the heat generated in the recovery boiler, compared to normal power boilers fuelled by coal, natural gas or oil.
The temperature limitations of 400° C. to 490° C. and 520° C. on the steam in a recovery boiler are not as strictly valid with bark originating from logs, but the fly ash from bark combustion in a bark boiler may also contain chlorine and potassium. As the sulfur content of bark is very low, chlorine reacts in the bark boiler with metals, which in turn may result in superheater corrosion. Calorimetric flow in bark is also much lower than in the black (“waste”) liquor flow, due to much smaller mass flow. This depends on pulp yield in pulping, and the amount of bark in the wood, from which the pulp is made.
WO 03/104547 and corresponding U.S. Published Application 2005/0252458 disclose a system for increasing the temperature and pressure of superheated steam produced at a recovery boiler plant of a pulp mill in such a way that no corrosion occurs or the rate of corrosion is at acceptable levels. WO 03/104547 and corresponding U.S. Published Application 2005/0252458 disclose a system in which the recovery boiler is provided with at least one cavity having walls formed of water-cooled tubes connected to the water/steam circulation system of the recovery boiler. The interior of the cavity is provided with a heat exchanger for final superheating of the steam generated in the recovery boiler, whereby the heat exchanger is connected to the superheaters of the boiler. The cavity is heated by burning fuel in such a manner that non-corrosive conditions in the superheater cavity are guaranteed. The fuel can be a gas produced by gasifying biomass. The basic feature of this system is that at least one combustion cavity is provided in connection with a recovery boiler for the final superheating of steam produced in the superheater section of the recovery boiler.